Circuit arrangements as described above are known. Oscillators in general and high frequency oscillators in particular are generally so constructed that a damped oscillator circuit is dedamped by an external circuit arrangement referred to herein as the dedamping circuit which compensates for the damping of the generated oscillation. In the ideal case the damping compensation should be such that the oscillations with a defined frequency should not be damped at all. The defined frequency in this context is the natural frequency or resonance frequency of the oscillator circuit. The exact dedamping required for the oscillator working point, at which it oscillates at its natural frequency, cannot technically be adjusted with the required accuracy due to environmental influences and due to circuit tolerances causing variations in the circuit parameters. Thus, for practical reasons the effect of the dedamping or of the respective feedback control is permitted to be larger than necessary with due regard to appropriate reserves. Such reserves are also necessary in order to assure the starting of the oscillator circuit with certainty. The oscillation amplitude increases until saturation or limitation effects prevent a further rise. These effects keep the amplitude at a determined value.
The measures taken under the heading of the so-called self-limiting oscillator principle, however, have a detrimental effect on the phase noise, particularly when the intrinsic or no load circuit quality of the oscillator circuit is low. Optimally low values for the phase noise can be achieved if the energy supplied to the oscillator circuit is equal to the energy used up by the oscillator circuit due to its intrinsic losses. If the oscillator circuit losses are high it follows that especially much energy must be supplied and that the energy reserves to be kept available are correspondingly very high.
Generally, the current consumption of the oscillator is of secondary importance, however it may not be disregarded. The current consumption is basically higher than normal in a limiting oscillator subject to limiting effects because the energies which are required for compensating such limiting effects must also be supplied in order to overcome the limiting effects. Additionally, in connection with CMOS oscillators cross-currents in a transition range must also be taken into account. The transition range is defined as a short time duration during which both complementary transistors are conducting.
An oscillator, contrary to plain amplifiers, is a feedback controlled amplifying system. Energy derived at the output of the oscillator is amplified and returned to the input of the oscillator. If the amplification is so large that the returned energy exceeds the damping losses, the amplitude at the output of the oscillator circuit will gradually increase. This increase may, for example be limited in that any component within the respective electric circuit reaches its power limit. Such an instance is referred to as self limitation of the oscillator. An oscillation thus produced does not have, as a rule, an optimal spectral purity. Such purity is impaired due to the occurrence and the size of side band noise.
In addition to self-limiting or self-regulating oscillators there are also known externally regulated oscillators wherein the amplification is adjusted manually in such a way that the amplitude of the generated oscillation does not exhaust the power reserves of the amplifier. The resulting feedback has a larger spectral purity than in the case of a self-limiting control. Ideally, the amplification should not fluctuate.